Voltammetric Perspectives on the Acidity Scale and H+/H2 Process in Ionic Liquid Media

Abstract

Nonhaloaluminate ionic liquids (ILs) have received considerable attention as alternatives to molecular solvents in diverse applications spanning the fields of physical, chemical, and biological science. One important and often overlooked aspect of the implementation of these designer solvents is how the properties of the IL formulation affect (electro)chemical reactivity. This aspect is emphasized herein, where recent (voltammetric) studies on the energetics of proton (H+) transfer and electrode reaction mechanisms of the H+/H2 process in IL media are highlighted and discussed. The energetics of proton transfer, quantified using the p Ka (minus logarithm of acidity equilibrium constant, Ka) formalism, is strongly governed by the constituent IL anion, and to a lesser extent, the IL cation. The H+/H2 process, a model inner-sphere reaction, also displays electrochemical characteristics that are strongly IL-dependent. Overall, these studies highlight the need to carry out systematic investigations to resolve IL structure and function relationships in order to realize the potential of these diverse and versatile solvents.